It took me quite a while to guess that the replacement character for Search & Replace is not `$1` or `\1` but `\1`. [STFW](https://en.wikipedia.org/wiki/RTFM) came up empty, since most questions are about problems with the pattern, not with the replacement/substitution character.
I think this should be documented somewhere on https://www.geany.org/manual/gtk/glib/glib-regex-syntax.html or `<local_path>/doc/geany/html/index.html#regular-expressions`.